BIOMECHANICS OF HIP JOINTSeminar by Dr. T. Vikram M.S ORTHO

Prathima institute of medical sciences karimnagar

INTRODUCTION

• The hip joint, or coxofemoral joint, is the articulation of the acetabulum of the pelvis and the head of the femur

• diarthrodial ball-and-socket joint• three degrees of freedom:1. flexion/extension in the sagittal plane2. abduction/adduction in the frontal plane3. medial/lateral rotation in the transverse plane

• The primary function of the hip joint is to support the weight of the head, arms, and trunk (HAT) both in static erect posture and in dynamic postures such as ambulation, running, and stair climbing.

STRUCTURE OF THE HIP JOINTProximal Articular SurfaceAcetabulum• The opening of the acetabulum is

approximately laterally inclined 50°; anteriorly rotated (anteversion) 20°; and anteriorly tilted 20° in the frontal, transverse, and sagittal planes, respectively

• Acetabular depth can be measured as the center edge angle of Wiberg

• Center edge angles are classified as follows:• definite dysplasia less than 16°• possible dysplasia 16° to 25° and • normal greater than 25°• In addition, abnormalities in acetabular depth, inclination,

and version (abnormal positioning in the transverse plane) can also affect femoral head coverage

• Anteversion of the acetabulum exists when the acetabulum is positioned too far anteriorly in the transverse plane

• Retroversion exists when the acetabulum is positioned too far posteriorly in the transverse plane

Acetabular labrum

• The entire periphery of the acetabulum is rimmed by a ring of wedge-shaped fibrocartilage called the acetabular labrum

• Deepens the socket, increases the concavity of the acetabulum, grasping the head of the femur to maintain contact with the acetabulum

• It enhances joint stability by acting as a seal to maintain negative intra-articular pressure

• Also provide proprioceptive feedback

Distal articular surface

• The head of the femuro fovea or fovea capitiso ligament of the head of the femur (ligamentum teres)

ANGULATION OF THE FEMUR• There are two angulations made by the head and neck of

the femur in relation to the shaft

Angle of inclination occurs in the frontal plane between an axis through the femoral head and neck and the longitudinal axis of the femoral shaft

Angle of torsion occurs in the transverse plane between an axis through the femoral head and neck and an axis through the distal femoral condyles

1.ANGLE IF INCLINATION OF FEMUR

• The angle of inclination of the femur approximates 125°

• Normal range from 110° to 144° in the unimpaired adult

• With a normal angle of inclination, the greater trochanter lies at the level of the center of the femoral head

• A pathological increase in the medial angulation between the neck and shaft is called coxa valga

• A pathological decrease is called coxa vara

• Both coxa vara and coxa valga can lead to abnormal lower extremity biomechanics altered muscle function,and gait abnormalities

2.ANGLE OF TORSION OF THE FEMUR

• The angle of torsion of the femur can best be viewed by looking down the length of the femur from top to bottom

• An axis through the femoral head and neck in the transverse plane will lie at an angle to an axis through the femoral condyles, with the head and neck torsioned anteriorly (laterally) with regard to an angle through the femoral condyles

• In the adult, the normal angle of torsion is considered to be 10° to 20°, 15° for males and 18° for females

• Femoral anteversion is considered to exist when angle of anterior torsion is greater than 15° to 20°

• A reversal of anterior torsion, known as femoral retroversion, occurs when angles are less than 15° to 20°

• Femoral anteversion is associated with increased medial rotation ROM and concurrent decreased lateral rotation so that the total excursion of hip rotation motion remains the same

• Anteversion of the femoral head reduces hip joint stability because the femoral articular surface is more exposed anteriorly

• The line of the hip abductors may fall more posterior to the joint, reducing the moment arm for abduction

• When the femoral head is anteverted, pressure from the anterior capsuloligamentous structures and the anterior musculature may push the femoral head back into the acetabulum, causing the entire femur to rotate medially

• The knee joint axis through the femoral condyles is now turned medially

• Medial rotation of the femoral condyles alters the plane of knee flexion/extension and results, at least initially,in a toe-in gait and a compensatory lateral tibial torsion develop

• An anteverted femur will also affect the biomechanics of the patellofemoral joint at the knee and of the subtalar joint in the foot

• The effect of femoral anteversion may also be seen at the knee joint

COXA VARA

• In adolescence, growth of the bone results in a more oblique orientation of the epiphyseal plate

• The epiphyseal obliquity makes the plate more vulnerable to shear forces at a time when the plate is already weakened by the rapid growth that occurs during this period of life

• Weight-bearing forces may slide the femoral head inferiorly, resulting in a slipped capital femoral epiphysis in case of coxa vara

• Disadvantage of increasing the bending moment along the femoral head and neck

• The increased shear force along the femoral neck will increase the predisposition toward femoral neck fracture

COXA VALGA• Coxa valga also decreases the amount of femoral articular

surface in contact with the dome of the acetabulum.

• As the femoral head points more superiorly, there is a decreasing amount of coverage from the acetabulum superiorly.

• Consequently, decreases the stability of the hip and predisposes the hip to dislocation

• The resulting need for additional abductor muscle force may predispose the joint to arthrosis or may functionally weaken the joint, producing energy-consuming and wearing gait deviations

ARTICULAR CONGRUENCE• In the neutral or standing

position, the articular surface of the femoral head remains exposed anteriorly and somewhat superiorly

• Articular contact between the femur and the acetabulum can be increased in the normal non-weight-bearing hip joint by a combination of flexion, abduction, and slight lateral rotation

HIP JOINT CAPSULE

• Both joint capsule and ligamentum teres provide stability of the hip joint during distractive forces

HIP JOINT LIGAMENTS• Iliofemoral ligament(Y ligament of Bigelow)

• Pubofemoral ligament

• Ischiofemoral ligament

STRUCTURAL ADAPTATIONS TO WEIGHT BEARING

• In standing or upright weightbearing activities, at least half the weight of the HAT (the gravitational force) passes down through the pelvis to the femoral head, whereas the ground reaction force (GRF) travels up the shaft.

• These two forces, nearly parallel and in opposite directions, create a force couple with a moment arm

• (MA) equal to the distance between the superimposed body weight on the femoral head and the GRF up the shaft.

• These forces create a bending moment (or set of shear forces) across the femoral neck

Trabecular system• The medial (or principal

compressive) trabecular system

• The lateral (or principal tensile) trabecular system

• Accessory (or secondary) trabecular systems

• zone of weakness

• The forces of HAT and the ground reaction force that act on the articular surfaces of the hip joint and on the femoral head and neck also act on the femoral shaft

• Ranges of passive joint motion typical of the hip joint :-

Flexion 90° with the knee extended and 120° when the knee is flexed

Hip extension 10° to 20°Abducted 45° to 50° Adducted 20° to 30°Medial and lateral rotations of the hip the typical range is

42° to 50°

Hip Joint MusculatureMovements Flexion : chiefly by psoas major, iliacus assisted by rectus femoris and sartorius Adductor longus assists in early flexion following full

extension

Extension : gluteus maximus and the hamstrings.

Abduction : gluteus medius and minimus assisted by sartorius,tensor fasciae latae and piriformis Action is limited by adductor longus,pubofemoral ligament

and medial band of ilio femoral ligament

Adduction : by adductor longus, adductor brevis and adductor fibers of adductor magnus

Lateral rotation : piriformis, obturator internus and externus, superior and inferior gemelli and quadratus femoris

assisted by the gluteus maximus

Medial rotation : the anterior fibers of the gluteus medius and gluteus minimus, tensor fasciae latae

Piriformis muscle was a lateral rotator at 0° of hip flexion but a medial rotator at 90° of hip flexion

MOTION OF PELVIS ON THE FEMUR

• Whenever the hip joint is weight-bearing, the femur is relatively fixed, and motion of the hip joint is produced by movement of the pelvis on the femur

Anterior and Posterior Pelvic Tilt

Anterior and posterior pelvic tilts are motions of the entire pelvic ring in the sagittal plane around a coronal axis

In the normally aligned pelvis, the anterior superior iliac spines (ASISs) of the pelvis lie on a horizontal line with the posterior superior iliac spines and on a vertical line with the symphysis pubis

Anterior and posterior tilting of the pelvis on the fixed femur produce hip flexion and extension

Hip joint extension through posterior tilting of the pelvis Hip flexion through anterior tilting of the pelvis

Lateral Pelvic Tilt

Lateral pelvic tilt is a frontal plane motion of the entire pelvis around an anteroposterior axis

In the normally aligned pelvis, a line through the anterior superior iliac spines is horizontal

In lateral tilt of the pelvis in unilateral stance, one hip joint (e.g., the left hip joint) is the pivot point or axis for motion of the opposite side of the pelvis (e.g., the right side) as that side of the pelvis elevates (pelvic hike) or drops (pelvic drop).

• If a person stands on the left limb and hikes the pelvis, the left hip joint is being abducted because the medial angle between the femur and a line through the anterior superior iliac spines increases.

• If a person stands on the left leg and drops the pelvis, the left hip joint will adduct because the medial angle formed by the femur and a line through the anterior superior iliac spines will decrease

Lateral Shift of the Pelvis

• With pelvic shift, the pelvis cannot hike; it can only drop.

• Because there is a closed chain between the two weight-bearing feet and the pelvis, both hip joints will move in the frontal plane in a predictable way as the pelvic tilt (or pelvic shift) occurs

Forward and Backward Pelvic Rotation

• Pelvic rotation is motion of the entire pelvic ring in the transverse plane around a vertical axis.

• Forward (anterior) rotation of the pelvis occurs in unilateral stance when the side of the pelvis opposite to the weight-bearing hip joint moves anteriorly from the neutral position

• Forward rotation of the pelvis produces medial rotation of the weight-bearing hip joint

• Backward (posterior) rotation of the pelvis occurs when the side of the pelvis opposite the weight-bearing hip moves posteriorly

• Backward rotation of the pelvis produces lateral rotation of the supporting hip joint

Pelvic Rotation in Gait

In normal gait, the pelvis forwardly rotates around the weight-bearing hip while the other limb prepares for or is in swing

Because this happens first on one leg and then on the other, it appears to the eye as if the pelvis is forwardly rotating and then backwardly rotating

Coordinated Motions of the Femur,Pelvis,and Lumbar Spine

Pelvifemoral Motion

When the femur, pelvis, and spine move in a coordinated manner to produce a larger ROM than is available to one segment alone, the hip joint is participating in what will predominantly be an open-chain motion termed pelvifemoral motion.

Also been referred to as pelvifemoral rhythm

2 types of response

• The open-chain response (the ability of each joint in the chain to move independently)

the head and trunk will follow the motion of the pelvis (moving the head through space)

• Closed-Chain responsethe head will continue to remain relatively upright and

vertical despite the pelvic motions

Moving the head and arms through space

If the goal is to bend forward to bring the hands (and head) toward the floor, isolated flexion at the hip joints (anteriorly tilting the pelvis on the femurs) is generally insufficient to reach the ground

Moving the foot through space

When a person is lying on the right side, the left foot may be moved through an arc of motion approaching 90°

The abducting limb is in an open chain

• A true open-chain response to isolated hip flexion would displace the head and trunk forward, with the line of gravity falling in front of the supporting feet

• In a functional closed chain, motion at the hip (one link in the chain) is accompanied by an essentially mandatory lumbar extension to maintain the head over the sacrum

• Compensatory motions of the lumbar spine that accompany given motions of the pelvis and hip joint in a functional closed chain

HIP JOINT FORCES AND MUSCLEFUNCTION IN STANCE

Bilateral Stance

• The line of gravity falls just posterior to the axis for flexion/extension of the hip joint

• In the frontal plane during bilateral stance, the superincumbent body weight is transmitted through the sacroiliac joints and pelvis to the right and left femoral heads

• joint axis of each hip lies at an equal distance from the line of gravity of HAT

• The gravitational moment arms for the right hip(DR) and the left hip (DL) are equal

• Because the body weight (W) on each femoral head is the same (WR = WL), the magnitude of the gravitational torques around each hip must be identical

• WR X DR =WL X DL

• The gravitational torques on the right and left hips occur in opposite directions.

• The weight of the body acting around the right hip tends to drop the pelvis down on the left (right adduction moment), whereas the weight acting around the left hip tends to drop the pelvis down on the right (left adduction moment)

• These two opposing gravitational moments of equal magnitude balance each other, and the pelvis is maintained in equilibrium in the frontal plane without the assistance of active muscles

• Assuming that muscular forces are not required to maintain either sagittal or frontal plane stability at the hip joint in bilateral stance, the compression across each hip joint in bilateral stance should simply be half the superimposed body weight (or one third of HAT to each hip)

• In bilateral stance when both lower limbs bear at least some of the superimposed weight, the contralateral abductors and adductors may function as synergists to control the frontal plane motion of the pelvis.

Unilateral stance • The left leg has been lifted from the ground and the full

superimposed body weight (HAT) is being supported by the right hip joint.

• The weight of the non-weightbearing left limb that is hanging on the left side of the pelvis must be supported along with the weight of HAT by right hip joint.

• Of the one-third of the portion of body weight found in the lower extremities, the non-weightbearing limb must account for half of that, or one sixth of the full body weight

• The magnitude of body weight (W) compressing the right hip joint in right unilateral stance, therefore, is

• Right hip joint compressionbody weight =[2/3 x W] + [1/6 x W]= 5/6 x W

• The force of gravity acting on HAT and the nonweightbearing left lower limb (HATLL) will create an adduction torque around the weight-bearing hip joint

• Gravity will attempt to drop the pelvis around the right weight-bearing hip joint axis.

• The abduction countertorque will have to be supplied by the hip abductor musculature

• The result will be joint compression or a joint reaction force that is a combination of both body weight and abductor muscular compression.

Compensatory Lateral Lean of the Trunk

• the compensatory lateral lean of the trunk toward the painful stance limb will swing the line of gravity closer to the hip joint, thereby reducing the gravitational moment arm

• It does reduce the gravitational torque

Use of a Cane Ipsilaterally Body wt passes mainly

through cane

Use of a Cane Contralaterally Cane assists the abductor

muscles in providing counter torque

Pathological Gaits

• When a lateral trunk lean is seen during gait and is due to hip abductor muscle weakness, it is known as a gluteus medius gait

• If the same compensation is due to hip joint pain, it is known as an antalgic gait

• If lateral lean and pelvic drop occur during walking, the gait deviation is commonly referred to as a Trendelenburg gait

Femoroacetabular Impingement

Cam impingement

Pistolgrip deformity of the femoral neck

Pincer impingement

caused by aberrations of the acetabulum

Reducing joint reaction forceReduced by

Reducing the body weight-generated momentum

By reducing body weight or reducing the body weight lever arm

Seen in Trelendenburg gait(leaning towards the diseased hip)

Reducing the required hip abductor force

Altering the neck-shaft angle through varus osteotomy/varus placement of the femoral stem

Increasing offset or medialization of the socket

Use of cane in contralateral hand

Biomechanics of total hip arthroplastyStability and range of

motion depends on :

1. Head size

2. Head-neck ratio and

3. Implant design

Increasing the head diameter increases the jumping distance(i.e., the radius of the femoral head)

Reduction of rates of revision for dislocation with increasing head size

Primary arc of the joint

Increase in head-neck ratio increases this arc, improving the range of motion

Head-neck ratio < 2:1 increase the risk of impingment leading to fixation failure, limited function and dislocation

Acetabular design features that can help increase the primary arc

Semicaptive sockets that are greater than hemisphere reduce the primary arc and may have paradoxically adverse effect on stability

Acetabular hemispherical component

1. If relatively small , stress transferred to the center

2. If relatively large , stress transferred to the periphery

Peripheral strains acting on a force vector perpendicular to the tangent at the rim stabilize the cup

Optimizing fixation and the low frictional torque arthroplasty

CHARNLEY conceptShorten lever arm of the body weight by deepening the

acetabulum and To lengthen the lever arm of the abductor mechanism by

reattaching the osteotomized greater trochanter laterally

Leading to decrease in moment produced by body weight there by reducing counterbalance force that the abductor mechanism must exert

Estimated load o femoral head in stance phase of gait is 3 times body weight

When standing on 1 leg abductor muscles work done is 2.5 times body weight

Load on femoral head during straight leg raising is 3 times body weight

While lifting, running, jumping 10 times body weight

Abductor lever arm shortened in arthritis and also where head is lost or neck is shortened

During the gait cycle, forces are directed against the prosthetic femoral head from a polar angle between 15-25 degrees anterior to the saggital plane of the prosthesis

Body’s center of gravity is posterior to the axis of the joint making the stem to deflect medially in coronal plane and posteriorly in saggital plane producing torsion of the stem

As seen in arising from chair, ascending and descending stairs, lifting

Reducing the joint reaction force in total hip arthroplasty

Lateralizing the femoral component by increasing the horizontal femoral offset

Inadequate restoration of offset results in hip abductor insufficiency and soft tissue laxity

Excessive femoral offset can also predispose to failure by overtightening of the hip, increasing the stress placed on the femoral fixation interface